Re:
Ok so you’ve got 2 options really,
Find a front to match your rear (sport lever, c2 caliper) shouldn’t be to hard but getting parts to service the c2 brake are getting harder to find but not impossible. I guess this will be down to whether you are looking for period correctness or not.
your No5 caliper is for 145r/165f rotors. That to me (I’m on the larger side and always go 180/200) is pretty small so be aware of that when looking for a front. One way to do it would be to buy a front brake with a number 3 caliper and then use that caliper on the back using a 165 rotor, then you need a post mount fork to IS caliper adaptor HBSM20mm. This “should” then allow you to fit a 185mm rotor on the front. I can probably check this (I have a spare c2 caliper and one of those mounts on a bike) in the next day or so if I get time.
Replace both front and rear with a more modern brake. Sticking with hope you are fine for spares for anything from the mono mini (black and gold) forward. Rotors will also be easier to find as the sizes changed a little bit and there are plenty of adapters for different configurations of frame/fork and caliper.